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Concept
Steroid ester
Summary
A steroid ester is an ester of a steroid. They include androgen esters, estrogen esters, progestogen esters, and corticosteroid esters. Steroid esters may be naturally occurring/endogenous like DHEA sulfate or synthetic like estradiol valerate. Esterification is useful because it is often able to render the parent steroid into a prodrug of itself with altered chemical properties such as improved metabolic stability, water solubility, and/or lipophilicity. This, in turn, can enhance pharmacokinetics, for instance by improving the steroid's bioavailability and/or conferring depot activity and hence an extended duration with intramuscular or subcutaneous injection.
Esterification of steroids with fatty acids was developed to prolong the duration of effect of steroid hormones. By 1957, more than 500 steroid esters had been synthesized, most frequently of androgens. The longer the fatty acid chain, up to a certain optimal length, the longer the duration when prepared as an oil solution and injected. Across a chain length range of 6 to 12 carbon atoms, a length of 9 or 10 carbon atoms (nonanoate or decanoate ester) was found to be optimal in rodents in the case of testosterone esters. Fatty acid esters increase the lipophilicity of steroids, with longer fatty acids resulting in greater lipophilicity. The greater solubility in oil allows the steroid esters to be dissolved in a smaller oil volume, thereby allowing for larger doses with intramuscular injection. In addition, the greater the lipophilicity of the steroid, as measured by the octanol/water partition coefficient (logP), the slower its release from the oily depot at the injection site and the longer its duration.
Steroid esters can also be prepared as crystalline aqueous suspensions. Aqueous suspensions of steroid crystals result in prolongation of duration with intramuscular injection similarly to oil solutions. The duration is longer than that of oil solutions, intermediate between oil solutions and subcutaneous pellet implants.
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